Field of the Invention
The present invention relates to operational amplifier type difference circuits, and more particularly to such circuits which are used as differential input-to-single-ended output converters and which have a capability to reject common mode signals.
Description of the Prior Art
The use of difference amplifiers employing operational amplifiers for the purpose of conversion of a differential input signal to a single-ended output signal, and the employment of the inherent characteristics of the circuit to reject any unwanted common mode signal is well known. Prior to the present invention, the range of amplitude of common mode signal rejection capability for such circuits with unity gain has generally been limited to peak-to-peak amplitudes approaching twice the difference of the positive and negative voltage supplies for the operational amplifier circuit.
While this range of rejection capability is adequate for many purposes there are applications for which a greater range of common mode rejection capability is desired. One such application is in conjunction with multi-frequency tone receiver equipment for telephonic call processing systems. In such equipments there is a possibility of presence of unwanted common mode signals in wire pairs designed to carry balanced, or differential signals, with the common mode signals reaching peak magnitudes as high as 50 volts.
Accordingly, an object of the present invention is to provide operational amplifier construction of difference amplifier circuit having a range of common mode signal rejection capability which is greatly extended beyond the nominal limit of peak-to-peak common mode signals equal to twice the power supply voltage magnitude for the operational amplifier circuit while retaining a gain for the differential signal equal to unity.
Another object is to provide a circuit in accordance with the previous object and further of a construction which may be adapted to different requirements by simple changes in resistive component values.